Electrical timers are an old and varied group of devices. They operate using a wide variety of techniques including mechanical, dashpot, thermal and electronic. The various types of timers have different advantages, as a function of cost, over each other in the areas of reliability, repeatability, life expectancy, size, recycle time, power consumption and versatility.
For modern uses, size has become an increaseingly important consideration. Small size units generally carry with them the additional advantages of lesser weight and smaller power consumption. The use of modern semiconductors can be a big advantage in reducing size, weight and power consumption. However, the advantages attendant upon the use of semiconductors are obtained only if they are operated within their electrical and thermal ratings. The thermal rating, in particular, becomes a critical factor when designing for small size. It becomes necessary to adequately dissipate the heat so as to prevent the detoriated performance that ocurrs when the thermal ratings are exceeded.
Accordingly, it is a major purpose of this invention to provide a small size, semiconductor based, electronic timer having heat dissipation characteristics which will maintain the semiconductors within their thermal ratings.
In many functions, a timer may be used repeatedly over a relatively short period of time. As the timer is used, the ambient temperature of the timing circuit tends to increase because of the heat generated. This may cause the time constant of the timing circuit to vary as a function of temperature.
Accordingly, it is another purpose of this invention to provide a timer design in which not only is the heat adequately dissipated but in which the temperature rise to which the timing circuit is subjected is minimized during a period of repeated use of the timer.
In most applications, a timer has to be designed so the user can set the switching time to the desired interval. Accordingly, it is a further purpose of this invention to achieve the above purposes in a design that permits the user to readily and simply adjust the switching time.
As is usually the case in industrical applications, it is an important purpose of the invention to achieve all the above results in an economical design that is efficient not only in its use of power but is also relatively easy to fabricate, simple to maintain and inexpensive to produce.
Thus, it can be said that an overall purpose of this invention is to provide an optimum trade off between the conflicting parameters of small size, high thermal dissipation, high versatility and low cost.